(a) Field of the Invention
The present invention relates to photonic transversal filters and more particularly to photonic transverse filters employing wavelength reuse to increase the number of taps and therefore the accuracy of the filters when apodized filter taps are being used for increasing the suppression of the filtered response amplitude at undesirable frequencies around the passband.
(b) Description of the Related Art
Photonic links are commonly used for signal distribution in antenna systems. It has also been established that RF-photonic links in so called fiber-radio networks have tremendous commercial potential for distributing microwave signals, especially those at mm-wave frequencies, between the base-station of a wireless network and the remoted sites of transmit/receive antennas.
In general, RF-photonic transversal filters enable one to accomplish an assortment of signal processing functions for microwave signals that have been modulated on optical carriers, as in fiber-remoted antenna systems. Transversal filters are finite impulse response (FIR) digital filters where for an impulse function input the filter response is finite and eventually dies down to zero. FIR filters are inherently stable, require no feedback, and can have linear phase.
The RF-photonic transversal filters can, for example, enhance the signal to noise ratio (SNR) in the receive chain of a multifunction antenna, while rejecting undesirable interferers or spurs. In these antenna systems, one exploits the broadband modulation capabilities of photonics to distribute the RF received at the sensor aperture to multiple locations, e.g. radar receivers or communications receivers, for further processing. Wavelength division multiplexing (WDM) laser-sources used to form the filters and microwave receivers are typically located in a secure (or benign) environment, while the optical modulator is mounted at the sensor aperture. As part of the antenna-remoting system, these filters serve to select a desired frequency segment of the RF modulated on the optical carrier, before it is distributed to an appropriate microwave receiver. Because each of these receivers is typically designed to cover a specific microwave band, for example the L-band that ranges from 1 to 2.6 GHz, acquiring more flexibility to reconfigure the filter's center frequency and passband width would enhance its signal processing versatility.
In addition, many of the signal processing algorithms, for example, ones that utilize digital signal processing DSP, employed in the microwave receivers are only exercised over relatively narrow frequency spans of, for example, less than 500 MHz. An agile RF-photonic filter with a sufficiently narrow passband can enable one to track a received signal with improved SNR over the small operation bandwidth of the digital processor. In order to achieve a narrow passband, a high side lobe suppression ratio (SLSR) is desirable that distinguishes the passband by suppressing the side lobes of the filter well below the −1 dB bandwidth of the passband.
Filter coefficients or filter taps of a photonic filter are obtained from wavelengths input to the filter. The wavelengths correspond to waves generated by single frequency laser sources whose outputs are input to the WDM of the filter. The larger the number of the wavelengths input to the filter, the larger the number of the taps produced by the filter, the narrower the passbands of the filter, and the higher the accuracy of the filter. However, generating a large number of taps involves a large number of laser sources that each output a wave of a particular wavelength. Reducing the number of laser sources is desirable because it reduces the number of components required by the filter. Wavelength reuse allows a filter to use fewer number of laser sources and still obtain the same number of filter taps. However, the number of taps, and therefore wavelength reuse to generate a higher number of taps, do not impact side lobe suppression. Consequently, the SLSR attained via conventional schemes that utilize wavelength reuse is too small to be of practical interest in the above applications.
Therefore, there is a need to achieve high filter accuracy while simultaneously obtaining narrow passbands and large side lobe reduction.